The present invention relates to an improved method of removing gaseous fluoride, condensable tars, particulate matter and the like from gases which evolve from a Hall-Heroult-type aluminum reduction cell, particularly the off-gases from a reduction cell with a Soderberg anode and prebaked anode baking furnaces.
The cell off-gases from an aluminum reduction cell generally comprise a dilute mixture of air with gaseous fluorides, carbon dioxide, carbon monoxide, particulate matter and the like. The gaseous fluorides are essentially HF. The particulate matter comprises finely divided alumina, carbon and other carbonaceous materials and also solid fluorides, such as cryolite (Na.sub.3 AlF.sub.6), aluminum fluoride (AlF.sub.6), sodium fluoride (NaF), calcium fluoride (CaF.sub.2) and chiolite (Na.sub.5 Al.sub.3 F.sub.4). Soderberg reduction cells have one large anode which is baked in place from a paste of carbon aggregate and pitch or tar. The baking of the anode results in the evolution of considerable amounts of tarry, carbonaceous materials, commonly termed "tar fog". The carbonaceous materials evolving from vertical stud Soderberg anodes are sometimes of sufficient concentration to allow for the combustion of the tarry materials, but, because of the harsh environment, maintaining burners in operating condition is very difficult. There is no practical way to economically burn the carbonaceous materials evolving from horizontal stud Soderberg anodes because of the low concentration of carbonaceous materials in the off-gases. The tar-contaminated gas mixture evolving from a Soderberg reduction cell renders the subsequent treatment of the gas very difficult because much of the carbonaceous matter in the gas is sticky or condensable and thus tends to foul or plug any subsequent gas treating facility.
Generally, two methods have been employed over the years in treating the cell off-gases from an aluminum reduction cell to remove fluorides. The first involves the scrubbing of the cell off-gases with water to remove the fluorides. However, the wet method is not very desirable because it more or less converts an air pollution problem into a water pollution problem due to the fact that an aqueous solution of fluoride is very difficult to discard without extensive treatment. Frequently, the wet method includes treatment with lime or limestone to react with the fluorides to form CaF.sub.2. The other method, a dry method, involves intimately contacting the cell off-gases with the alumina fed to the cell so as to sorb the gaseous fluoride in the cell off-gases onto the alumina surfaces. Up to 99.95% of the gaseous fluoride evolving from the cell can be captured by this method. An additional advantage of the dry method is that all of the fluoride captured can be returned to the cell along with the cell feed. Several methods have been employed in contacting the alumina cell feed with the fluoride laden cell off-gases. One method shown in Canadian Pat. No. 613,352 and U.S. Pat. No. 2,875,844 is to introduce alumina into the moving stream of the cell off-gases and then subsequently removing the particulate matter including the alumina from the gaseous stream by a suitable means, such as a baghouse or an electrostatic precipitator. Another method shown in U.S. Pat. Nos. 2,934,405 and 3,503,184 is to pass the fluoride-containing off-gases through a bed of alumina. In the latter method, probably the most efficient, it is preferred to pass the fluoride-laden cell off-gases through a bed of finely divided alumina and then subsequently removing any particulate matter including alumina by means of filter bags or an electrostatic precipitator. However, neither of these two dry methods have been employed to any significant extent with the off-gases from a horizontal stud Soderberg cell because the tarry, carbonaceous materials which evolve from the baking anode tend to foul and plug up any gas treatment equipment facilitates that might be employed.
In the manufacture of carbon anodes for use in a prebake aluminum reduction cell, the anodes are usually baked in a ring furnace such as is described in U.S. Pat. No. 2,699,931 which is hereby incorporated in its entirety by reference. In these anode baking furnaces, the pitch which holds the "green" anode together is carbonized to provide the density, conductivity and soundness for use in an aluminum reduction cell. In addition to the volatile hydrocarbon material which evolves, fluorides also evolve. The fluoride source in this instance is the carbon aggregate which is used to make up the green carbon anode. Some of the aggregate comes from used anode butts which are crushed and added to the mixture which makes up the anode. Much fluoride from the reduction cell bath remains on the surface and within the interstices of the anode butts. Tars and condensable carbonaceous materials also are generated in the combustion of fuel utilized to heat the furnace.
Prior methods of treating of effluent from these anode baking furnaces or ring furnaces as they are frequently called have generally been directed to burning the tars and carbonaceous materials. Except for passing the gas through a water spray chamber, no effort was usually made to remove the fluoride in the effluent.
Against this background, the present invention was developed.